the basic character of hydrides decreases as follows:
NH3>PH3>AsH3>SbH3>BiH3

all hydrides of group 15 element possesses a lone pair of electrons on central atom and thus behave as lewis bases..
NH3 is basic and forms salt with strong and weak acids...PH3 is a very weak base(may even be regarded as neutral)and thus forms salt with anhydrous hydrochloric acid gas...AsH3,SbH3 and BiH3 are not at all basic...
the reason for such trend is : as the size of central atom increases..the lone pair of electron occupies a larger volume ..in other words electron density on central atom decreases and consequently its tendency to donate a pair of electron decreases and hence its basic strength decreases as we move from NH3 to BiH3..
as basic strength is in that order so acidic strength order will be the reverse of that:
BiH3>SbH3>AsH3>PH3>NH3

Dear Student,
Please find below the solution to your problem.

(i) the basic character of hydrides decreases as follows:
NH3 > PH3 > AsH3 > SbH3 > BiH3
all hydrides of group 15 element possesses a lone pair of electrons on central atom and thus behave as lewis bases..
NH3 is basic and forms salt with strong and weak acids...
PH3 is a very weak base(may even be regarded as neutral)and thus forms salt with anhydrous hydrochloric acid gas...
AsH3,SbH3 and BiH3 are not at all basic...
the reason for such trend is : as the size of central atom increases..
the lone pair of electron occupies a larger volume ..

(ii) Due to inert pair effect, as we move down the group stability of pentahalides decreases.
So order isP > Sb > As > Bi for given X andF > CI > Br for given element because as size of halide increases, steric hinderance increases and molecule become unstable.

(iii)
Nitrogen not show allotropy due to its weak N-N single bond.
Therefore ability of nitrogen to form polymeric structure is less.

Thanks and Regards